Renewable bio-based polymers and composites derived from natural resources are generating_great interest due to depleting fossil fuel resources and the negative environmental impact of fossil fuel-based plastic products. Many bio-based polymers have been developed which show great potential for different applications; however, processing and_physical properties of these polymers are still not sufficient for many end-user applications. The poor processing and physical properties of these polymers can generally be attributed to low molecular weight, low crystallinity, and high moisture uptake. Improving the processability and physical properties of sustainable bio-based polymers is essential to make them suitable for different end-user applications. For example, polylactide is one of the most important bio-based polymers and it shows great potential in packaging, drug delivery, and biological scaffolds applications. However, the mechanical and processing properties of polylactide alone is not adequate for many of these applications. Crystallization increases the mechanical properties of the polymer.
Polylactide polymers (PLA, sometimes referred to as polylactic acid) are of increasing interest because they can be prepared from annually renewable resources such as corn sugars rather than oil or natural gas feedstocks. PLA resins are also capable of degrading rapidly under some composting conditions to regenerate carbon dioxide. The ability to compost these materials can provide more disposal options for these resins, compared to most other organic polymers. As a result, PLA resins are finding uses in a variety of packaging applications. These packaging applications include a variety of rigid and semi-rigid articles such as clamshell containers, deli and other food service trays and bottles. These packaging products are made mainly by extruding a sheet of the PLA resin and then thermoforming it.
PLA has certain characteristics which greatly affect how it is processed and the types of end-products that can be made from it. For example, the glass transition temperature (Tg) of PLA is only about 60° C., which is significantly lower than those of commonly available, high-volume polymers that have use temperatures such that during use they are in the glassy state. This low Tg means that parts made from PLA resins tend not to be very heat-resistant, as even moderately elevated temperatures are sufficient to induce a phase transition and soften the polymer.
One way to improve the heat resistance of a PLA resin is to partially crystallize it. A very significant improvement in heat resistance is seen when 20 J/g or more of crystallinity is induced in a PLA article. This can be done by annealing the part between the Tg and the crystalline melting temperature (Tm) of the resin. However, another characteristic of PLA resins is that under quiescent conditions they crystallize very slowly compared to most other common semi-crystalline polymers. This slow crystallization is a practical problem in many manufacturing processes, because the slow crystallization rates lead to very long cycle times. Equipment utilization is decreased and operating expenses are increased due to the slow crystallization rates.
Thermoforming is almost always limited to producing thin-wall articles that can be formed from a starting sheet material. Stretch blow molding processes are limited to producing certain types of hollow articles. In each case, thickness of the resulting parts is restricted. In addition, these processes are not amenable to forming complexly shaped parts.
Injection molding is a method in which thicker, more complex parts can be made. Injection molding starts with a molten polymer which is injected into the mold, and there is no simple way to stretch the polymer (to induce crystallization) within the mold in an injection molding process. Therefore, whereas crystallization is promoted by stretching the part to orient the polymer chains, crystallization must take place in an injection molding process without the benefit of stretching the part. In the injection molding process, quiescent crystallization dominates, rather than stress-induced crystallization.
Therefore, PLA resins have been injection molded, but with only limited ability to form parts which are stable at elevated temperatures. The conventional injection molding process for PLA uses a cold mold, which is at or below the glass transition temperature of the PLA resin. In order to produce reasonable cycle times, the polymer is quenched in the mold by rapidly cooling it to below its Tg, so it hardens enough to be demolded. Little crystallization can occur in this process, especially because of the inherently slow crystallization of PLA under quiescent conditions, and so the molded part is not very resistant to elevated temperatures.
If better heat resistance and mechanical properties are desired, it becomes necessary for the PLA resin to become more highly crystallized. This can be done on injection molded parts after they have been demolded, by conducting an annealing step during which the PLA resin is heated to about 70° C. to 130° C. for a period of time. This annealing step can cause the part to warp or shrink if the part is not constrained. In addition, this increases manufacturing costs substantially, and correspondingly higher equipment and energy costs are incurred. It is better to promote PLA crystallization while the part is still in the mold. This can be done by bringing the temperature of the part to 70° C. to 130° C. for a period of time before the part is demolded.
However, because PLA is inherently slow to quiescently crystallize, and because there is no possibility to stretch the polymer, it takes a long time for the part to develop the wanted crystallinity. Furthermore, the part tends to be softer at the higher temperatures needed to crystallize it within the mold, because it is kept above its Tg. This can make the part more difficult to eject, because of sticking to the mold and the possibility of distorting the part as it is removed. For these reasons, cycle times become very long in injection molding processes, if it is attempted to perform the crystallization step while the part is in the mold with conventional PLA molding compositions. PLA resins for injection molding processes generally have moderate molecular weights (such as a weight average molecular weight of 100,000 g/mol or less), to facilitate the process through a lower melt viscosity and faster crystallization rates. However, crystallization rates are still too slow to provide economically feasible cycle times.
A shorter cycle time is needed for the process to be economically viable. Various nucleating agents and plasticizers have been used in some processes, in order to increase the crystallization rate of PLA. Among these are materials such as talc, ethylene bis(stearamide), polyethylene glycol, acetyl-tributyl citrate, and tributyl citrate. These can provide various levels of improvement in manufacturing processes which require the polymer to be crystallized quiescently. However, these have not been found to reduce cycle times sufficiently, and manufacturing rates remain slow in an injection molding process. In general, these nucleating agents work for cold crystallization, in which the polymer and nucleating agent are heated to about 80-140° C. and the crystallization occurs in the solid state. It is only very rare that PLA crystalizes from the melt, even with nucleating agents.
Therefore, it is desirable to provide a PLA resin composition which can be processed rapidly in an injection molding process, to produce a crystalline or semi-crystalline molded product. Crystallization from the melt is important for injection molding, allowing short cycle times and no need to reheat the product for cold crystallization.
In addition to improved heat stability, PLA with higher crystallinity is expected to be less susceptible to hydrolytic degradation of the polymer and have better mechanical properties for some applications. It is especially desirable to provide bio-based nucleating agents for PLA.
Nucleation is important to a wide range of polymers, not just PLA. For example, nucleation is important in other biopolymers such as polycaprolactone (PCL) and polyhydroxybutyrate (PHB), among others, and non-biopolymers such as polyethylene (PE) and polypropylene (PP), among others.
Therefore, it is also desired to provide nucleating agents that are applicable across a range of polymers for which crystallization is beneficial.